Nitride semiconductor device and method of manufacturing the same

ABSTRACT

In the nitride semiconductor device using the silicon substrate, the metal electrode formed on the silicon substrate has both ohmic contact property and adhesion, so that the nitride semiconductor device having excellent electric properties and reliability is obtained. The nitride semiconductor device includes a silicon substrate ( 2 ), a nitride semiconductor layer ( 10 ) formed on the silicon substrate ( 2 ), and metal electrodes ( 8, 8′ ) formed in contact with the silicon substrate ( 2 ). The metal electrodes ( 8, 8′ ) has first metal layers ( 4, 4′ ) which are formed in a shape of discrete islands and in contact with the silicon substrate ( 2 ), and second metal layers ( 6, 6′ ) which are in contact with the silicon substrate ( 2 ) exposed among the islands of the first metal layers ( 4, 4′ ) and are formed to cover the first metal layers ( 4, 4′ ). Further, the second metal layers ( 6, 6′ ) are made of a metal capable of forming ohmic contact with silicon, and the first metal layers ( 4, 4′ ) are made of an alloy containing a metal and silicon, in which the metal is different than that in the second metal layer ( 6,6′ ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor device using asilicon substrate and method of manufacturing the nitride semiconductordevice, and more particularly, relates to a nitride semiconductor devicehaving an electrode of excellent ohmic contact properties and adhesionwith respect to the silicon substrate and method of manufacturing thenitride semiconductor device.

2. Description of the Related Art

A nitride semiconductor device is capable of producing high-output,short-wavelength light and has widely been used for a blue or green LEDand for a white LED, in which a fluorescent material is used incombination of a nitride semiconductor device. In addition, a nitridesemiconductor device has been intensively studied also as a high-speedelectric device such as HEMT.

When manufacturing a nitride semiconductor device, a nitridesemiconductor layer is generally grown by hetero-epitaxial growth on aforeign substrate such as sapphire, because a GaN substrate forhomo-epitaxial growth of a nitride semiconductor layer is expensive.However, obtaining a wafer having a large diameter is difficult even byusing a foreign substrate such as sapphire, nor at a lower cost. Also,since sapphire is insulative, an electrode cannot be formed on the backsurface of a sapphire substrate and electrodes of different polaritiesare needed to be formed at the same side of the nitride semiconductorlayer. Thus, a uniform current distribution within the nitridesemiconductor device is difficult to obtain.

Therefore, consideration has been made about fabrication of a nitridesemiconductor component on a silicon substrate, which is conductive andan electrode can be formed on the back surface thereof, and a waferhaving a large diameter is available in the market at a lower price.

For example, described in JP 2002-208729A is a nitride semiconductordevice fabricated by forming a metal-compound region having gallium,indium and silicon as its major components and an aluminum nitride layeron an n-type silicon substrate, then forming a nitride semiconductorcomponent structure thereon. On the back surface of the n-type siliconsubstrate, there is formed a cathode electrode made of vacuum-depositedtitanium and aluminum. On the p-side of the nitride semiconductor layer,there is formed an anode electrode made of vacuum-deposited nickel andgold. Also described therein is a formation of an electrode made ofsequentially stacked titanium/an alloy of gold, germanium andnickel/gold as substitute for the cathode electrode made of titanium andaluminum.

Described in JP 2003-8061A is a nitride semiconductor device in which alayer containing a IIIB group element is formed on an n-type siliconsubstrate and then a nitride semiconductor layer is formed thereon. Onthe back surface of the n-type silicon substrate, there is formed abonding electrode made of any one of Al, Ti, Zr, Hf, V and Nb. On thep-side of the nitride semiconductor layer, there is formed a thin-filmtransparent electrode made of any one of Ta, Co, Rh, Ni, Pd, Pt, Cu, Agand Au, and further formed a second bonding electrode.

Further, described in JP 2005-108863A is a nitride semiconductor devicein which a nitride semiconductor layer grown on a foreign substrate suchas sapphire is bonded to a silicon substrate via a conductive bondinglayer such as an eutectic crystal layer, then the foreign substrate suchas sapphire is removed to obtain the nitride semiconductor device.

However, when a nitride semiconductor device is formed by using asilicon substrate as described in JP 2002-208729A, JP 2003-8061A, and JP2005-108863A, a metal electrode formed on the silicon substrate is easyto detach during the manufacturing process of the device. That is,because a silicon substrate is conductive, it is possible to form ametal electrode on the back surface of the substrate, but the siliconsubstrate and metal electrode are needed to establish good ohmiccontact. However, when a metal electrode is made of a metal having agood ohmic contact with a silicon substrate, the metal electrode maydetach from the silicon substrate during the manufacturing process ofthe nitride semiconductor device such as a dicing process. In a nitridesemiconductor device, a nitride semiconductor layer having differentthermal expansion coefficient and lattice constant is needed to beformed on a silicon substrate. Therefore, stress loaded on the metalelectrode formed on the silicon substrate is larger than that in asilicon semiconductor device. Consequently, it is presumed thatdetachment of the metal electrode is particularly easy to occur.

Particularly, in the cases where the silicon substrate and the nitridesemiconductor layer are coupled by bonding as described in JP2005-18863A, the growth substrate is removed after bonding three layersof a growth substrate, a nitride semiconductor layer, and a siliconsubstrate, each having a different thermal expansion coefficient. Thiscauses a significant change in the warpage of the silicon substrateduring manufacturing process, resulting in increased occurrence ofdetachment of the metal electrode. In addition, after bonding thesilicon substrate and the nitride semiconductor layer by using aconductive bonding layer as described in JP 2005-18863A, annealingcannot be applied at a higher temperature than the melting point of theconductive bonding layer. Thus, the metal electrode formed on a siliconsubstrate after bonding needs to be made of a metal capable ofestablishing ohmic contact without being subjected to annealing at ahigh temperature, limiting the selection of the electrode materials.Therefore, satisfying both ohmic contact property and adhesion in themetal electrode becomes further difficult.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nitride semiconductordevice with excellent electric properties and reliability, by achievingboth ohmic contact property and adhesion to a substrate in a metalelectrode formed on a silicon substrate in a nitride semiconductordevice using a silicon substrate, and a method of manufacturing thesame.

To achieve the above-described object, a nitride semiconductor deviceaccording to one aspect of the present invention includes a siliconsubstrate, a nitride semiconductor layer formed on the siliconsubstrate, and a metal electrode formed in contact with the siliconsubstrate. The metal electrode has a first metal layer which is incontact with the silicon substrate and is formed in a shape of discreteislands, and a second metal layer which is in contact with the siliconsubstrate exposed among the islands of the first metal layer and isformed to cover the first metal layer. The second metal layer is made ofa metal capable of forming an ohmic contact with silicon, and the firstmetal layer is made of an alloy containing a metal and silicon, in whichthe metal is different than that in the second metal layer.

According to the present invention, high adhesion with the siliconsubstrate can be obtained by the first metal layer while maintaining agood ohmic contact with the silicon substrate by the second metal layer.Thus, a metal electrode excellent in both ohmic contact property andmechanical stability can be easily obtained. That is, in the presentinvention, the second metal layer is made of a metal such as platinumgroup metal capable of forming good ohmic contact with silicon. Althoughadhesion to the silicon substrate is not strong, the second metal layerforms good ohmic contact with the surface of the silicon substrateexposed among the discrete islands of the first metal layer. Incontrast, the first metal layer is made of an alloy containing a metaland silicon, in which the metal is different than that in the secondmetal layer, and has strong adhesion to the silicon substrate. Thus,when the first metal layer is formed in a shape of discrete islands onthe silicon substrate, the first metal layer serves as an anchor toconnect the silicon substrate and the second metal layer, so that theadhesion of the entire metal electrode can be ensured.

In the present invention, the expression “a metal different than that inthe second metal layer” means a metal different than a major componentof the second metal layer, but a metal contained in a small amount inthe second metal layer is not excluded. Also in the invention, theexpression “major component” means a component that is contained in anamount of 50% or more by molar ratio.

The first metal layer is preferably made of an alloy containing atransition metal and silicon, and more preferably, an alloy containingat least one selected from Ti, W, and Co, and silicon. The siliconalloys as described above have a high bonding strength to siliconsubstrate so that detachment of the metal electrode can be preventedmore efficiently.

The first metal layer is for ensuring adhesion to the silicon substrateand ohmic contact is formed between the silicon substrate exposed amongthe first metal layers and the second metal layer. Therefore, thediameter of the islands of the first metal layer is preferably 100 Å orless. With this arrangement, the ohmic contact area between the secondmetal layer and the silicon substrate increases and the contactresistance can be further reduced. In addition, the thickness of thefirst metal layer is preferably in a range from 5 to 100 Å. When thethickness is set as described above, by sputtering, the first metallayer can be formed in a shape of islands each having a microscopicdiameter.

When the silicon substrate is of p-type, the second metal layerpreferably contain at least one element of platinum group (Ru, Rh, Pd,Os, Ir, and Pt) as a major component, and most preferably contain Pt asa major component. The platinum group metals can easily form ohmiccontact with a p-type silicon substrate, and for example, it is possibleto form good ohmic contact even when the resistivity is somewhat high.Particularly, Pt can form good ohmic contact even when the resistivityof the p-type silicon substrate is 2 Ω cm or more. Also, the platinumgroup metals are preferable because if the resistivity of the siliconsubstrate is somewhat low, e.g. 0.5 Ω cm or less, ohmic contact can beformed without a high temperature annealing.

Further, the silicon substrate and the nitride semiconductor layer arepreferably bonded by using a conductive bonding layer. With thisarrangement, anti-detachment effect of the present invention becomesmore significant.

A method of manufacturing a nitride semiconductor device of the presentinvention concerns a method of manufacturing a nitride semiconductordevice having a silicon substrate, a nitride semiconductor layer formedon the silicon substrate, and a metal electrode formed in contact withthe silicon substrate. The method includes a process of forming a metalelectrode that includes a step of forming a first metal layer on thesilicon substrate so that the first metal layer is in contact with thesilicon substrate and is formed in a shape of discrete islands, and astep of forming a second metal layer to be in contact with the siliconsubstrate exposed among the islands of the first metal layer and tocover the first metal layer, the second metal layer is made of a metalcapable of establishing ohmic contact with silicon and the first metallayer is made of an alloy containing a metal and silicon, in which themetal is different than that in the second metal layer.

According to the present method of manufacturing, a first metal layerhaving a high adhesion to a silicon substrate is formed on a siliconsubstrate and subsequently, a second metal layer having good ohmiccontact property with a metal electrode is formed thereon. Thus, a metalelectrode excellent in both ohmic contact property and mechanicalstability can be formed.

As described above, according to the present invention, in a nitridesemiconductor device using a silicon substrate, a metal electrode formedon the silicon substrate has both ohmic contact property and adhesion sothat a nitride semiconductor device having excellent electricalproperties and reliability can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view showing an example of a nitride semiconductordevice according to the present invention.

FIG. 1B is a cross-sectional view taken along line B-B′ in FIG. 1A.

FIG. 2 is a schematic view showing a silicon substrate and a metalelectrode.

FIG. 3 is a cross-sectional view showing a first variation of thenitride semiconductor device.

FIG. 4 is a cross-sectional view showing a second variation of thenitride semiconductor device.

FIG. 5A is a sectional view illustrating a part of a manufacturingprocess of a nitride semiconductor device.

FIG. 5B is a cross sectional view illustrating a following process ofFIG. 5A.

FIG. 5C is a cross sectional view illustrating a following process ofFIG. 5B.

FIG. 5D is a cross sectional view illustrating a following process ofFIG. 5C.

FIG. 5E is a cross sectional view illustrating a following process ofFIG. 5D.

FIG. 5F is a cross sectional view illustrating a following process ofFIG. 5E.

FIG. 5G is a cross sectional view illustrating a following process ofFIG. 5F.

FIG. 5H is a cross sectional view illustrating a following process ofFIG. 5G.

FIG. 6 is a flow chart illustrating a part of a manufacturing process ofa nitride semiconductor device.

DENOTATION OF NUMERALS

-   1: nitride semiconductor deice-   2: silicon substrate-   4, 4′: first metal layers-   6, 6′: second metal layers-   8, 8′: metal electrodes-   10: nitride semiconductor layer-   12: p-electrode-   14: insulating protective film 16: n-electrode-   17: conductive bonding layer-   18: second insulation protective film-   20: growth substrate-   22: first main surface (front surface) of silicon substrate-   24: second main surface (back surface) of silicon substrate

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now preferred embodiments of the present invention will be describedbelow with reference to attached drawings. FIG. 1A and FIG. 1B are topview and cross-sectional view, respectively, showing a nitridesemiconductor device according to the present embodiment. In the nitridesemiconductor device 1 shown in FIGS. 1A and 1B, a nitride semiconductorlayer 10 is bonded on a front surface (first main surface) 22 of asilicon substrate 2 which is conductive and approximately rectangular,via a metal electrode 8, a substrate-side adhesion layer 9, asubstrate-side barrier layer 11, a conductive bonding layer 17, asemiconductor-side barrier layer 13, a semiconductor-side adhesion layer15, and a p-electrode 12. Further, a metal electrode 8′ is also formedon the back surface (second main surface) 24 of the silicon substrate 2.The p-electrode 12 is ohmically connected to the p-side of the nitridesemiconductor layer 10, and can be electrically connected to outside viathe silicon substrate 2. In addition, an insulating protective film 14is formed around the p-electrode 12. Meanwhile, an n-electrode 16 isformed at the n-side of the nitride semiconductor layer 10 and coveredby a second insulating protective film 18 except a pad portion 16 a. Then-electrode 16 can be electrically connected to outside through a wireand the like connected to the pad portion 16 a.

The nitride semiconductor device 1 shown in FIG. 1A and FIG. 1B uses aconductive silicon substrate 2 and has the metal electrode 8′ on theback surface 24 of the silicon substrate 2, so that electric current canbe supplied uniformly throughout the device. In addition, the siliconsubstrate is inexpensive and it enables the production of the devices ata low cost. Further, the silicon substrate 2 is an inherentlyeasily-diced material, and moreover, detachment of the metal electrodes8 and 8′ during dicing, which has been a problem, is prevented accordingto the present embodiment, so that high-yield production can berealized.

FIG. 2 is a schematic view showing a part of the metal electrode 8 onthe silicon substrate 2. As shown in FIG. 2, in the present embodiment,the metal electrode 8 formed on the front surface 22 of the siliconsubstrate 2 includes a first metal layer 4 formed into discrete islands,and a second metal layer 6 covering the first metal layer 4 and planarlyformed on the substantially entire surface of the silicon substrate 2.The second metal layer 6 is made of a metal such as platinum group metalcapable of forming good ohmic contact with silicon, and ohmicallyconnected to the front surface 22 of the silicon substrate 2 which isexposed among the discrete islands of the first metal layer 4. Incontrast, the first metal layer 4 is made of an alloy containing a metaland silicon, in which the metal is different than that in the secondmetal layer 6, and strongly adheres to the silicon substrate 2. Thefirst metal layer 4 serves as an anchor to connect the silicon substrate2 and the second metal layer 6. Therefore, with the metal electrode 8 ofthe present embodiment, detachment of the second metal layer 6 from thesilicon substrate 2 can be prevented efficiently due to anchor effect ofthe first metal layer 4, while securing good ohmic contact with thesilicon substrate 2 by the second metal layer 6. In addition, it is alsopreferable that the metal electrode 8′ formed on the back surface 24 ofthe silicon substrate 2 has the same composition and structure as thatof the metal electrode 8 on the surface. The following description onthe metal electrode 8 can also be applied to the metal electrode 8′.

It is preferable that if a metal electrode 8 having a first metal layer4 and a second metal layer 6 is disposed on the front surface 22 of thesilicon substrate 2 as shown in FIG. 3, detachment of the metalelectrode 8 at the time of dicing can be prevented. That is, detachmentbetween the silicon substrate 2 and the nitride semiconductor 10 can beprevented. Further, ohmic contact between the front surface 22 of thesilicon substrate 2 and the metal electrode 8 can also be securelyformed.

Similarly, It is preferable that if a metal electrode 8′ having a firstmetal layer 4′ and a second metal layer 6′ is disposed on the backsurface 24 of the silicon substrate 2 as shown in FIG. 4, detachment ofthe metal electrode 8′ at the time of dicing can be prevented. Further,ohmic contact between the surface 24 of the silicon substrate 2 and themetal electrode 8′ can also be established.

It is most preferable if the metal electrodes 8, 8′ are disposed on boththe front surface 22 and the back surface 24 of the silicon substrate 2as shown in FIG. 2. With this arrangement, both the front surface 22 andthe back surface 24 of the silicon substrate 2 can be provided with themetal electrode 8, 8′ having excellent ohmic contact property andadhesion.

The first metal layer 4 contains a metal and silicon in which the metalis different than that in the second metal layer 6. A silicon alloygenerally has high adhesion to silicon, so that the first metal layer 4has a high adhesion to the silicon substrate. In addition, silicon alloyalso has higher adhesion than that of silicon to a metal such as aplatinum metal having excellent ohmic contact property with silicon.Further, mechanical adhesion between the first metal layer 4 and thesecond metal layer 6 can also be obtained by enclosing the periphery ofthe first metal layer 4 formed in a shape of discrete islands with thesecond metal layer 6. Thus, when the first metal layer 4 is formed in ashape of discrete islands on the silicon substrate 2, the first metallayer 4 serves as an anchor to connect the silicon substrate 2 and thesecond metal layer 6, so that the detachment of the entire metalelectrode 8 can be prevented effectively.

Herein, the silicon alloy expressed herein as “alloy containing a metaland silicon, in which the metal is different from that in the secondmetal layer” is not specifically limited, as long as the alloy canstrongly adhere to the silicon substrate 2. “Metal different than thatin the second metal layer” is, however, preferably a transition metal,and more preferably, one selected from the group consisting of Ti, Co,W, and Ni. Among them, Ti, Co, or W is preferable and Ti is mostpreferable. These silicon alloys show high connection strength to thesilicon substrate 2. In addition, the silicon content in the first metallayer 4 is preferably 40 to 75 atom %. When the silicon content is lessthan 40 atom %, the connection strength with the silicon substrate 2 islikely to decrease. When the silicon content is higher than 75 atom %,the anchor effect of the first metal layer 4 to the second metal layer 6is likely to decrease. The expression “alloy” in the presentspecification means a material made by adding and melting a differentmetal element or non metal element to a metal, and includes silicide. Itis further preferable if the first metal layer 6 is silicide.Particularly, TiSi₂ is most stable silicide and therefore preferable.

On the other hand, the second metal layer 6 is made of a metal capableof forming good ohmic contact with silicon. When the silicon substrate 2is of p-type, the second metal layer 6 is preferably made of a metalhaving a large work function. For example, the second metal layer 6 ispreferably made of a platinum group metal (Ru, Rh, Pd, Os, Ir, or Pt),and most preferably Pt. Platinum group metals can easily form ohiccontact with a p-type silicon substrate 2, and for example, a good ohmiccontact can be formed even when the resistivity of the silicon substrateis somewhat high. Particularly, Pt is capable of forming good ohmiccontact even when the resistivity of the p-type silicon substrate 2 is 2Ωcm or greater. In addition, when the resistivity of the p-type siliconsubstrate 2 is somewhat low, a platinum group metal can form ohmiccontact without a high temperature annealing. On the other hand, whenthe silicon substrate is of n-type, a metal containing Ti or Al as amain component is preferable, and Ti or Al is more preferable. Inaddition, the second metal layer 6 needs to be made of a metal capableof forming good ohmic contact as a whole. For example, a platinum metalmay contain another metal element to a degree that does not impair itsohmic contact property.

If the anchor effect due to the first metal layer 4 can be obtained, thefirst metal layer 4 and the second metal layer 6 are not necessarily incontact with each other. For example, a third metal layer, which canadhere tightly to the both the first metal layer 4 and the second metallayer 6, may be interposed therebetween.

The metal electrode 8 may be formed as below. First, the first metallayer 4 is grown in a shape of discrete islands on the silicon substrate2. The first metal electrode 4 can be formed in a shape of islands byway of patterning using photolithography, but a shape of islands ispreferably formed by using sputtering or vacuum deposition, andparticularly, sputtering is preferable. That is, in sputtering,island-shaped cores are formed in a cluster in the initial stage ofgrowing. As the growth proceeds, the clusters joined each other to forman uniform film. Therefore, if the sputtering is stopped before theuniform film is grown, the first metal layer 4 of discrete islands eachhaving a microscopic diameter can be formed. Then, the second metallayer 6 is formed to cover the first metal layer 4 formed in the shapeof discrete islands. The second metal layer 6 can be formed by using aconventional method such as sputtering and vacuum deposition. Thus, themetal electrode 8 is formed on the silicon substrate 2.

The first metal layer 4 is to secure the adhesion to the siliconsubstrate 2, and ohmic contact is formed between the silicon substrate 2and the second metal layer 6 which is exposed among the first metallayer 4. Therefore, the diameter of each island of the first metal layer4 is preferably 100 Å or less. With this arrangement, the ohmic contactarea between the second metal layer 6 and the silicon substrate 2increases and the contact resistance therebetween further decreases. Inaddition, an average pitch of the islands, i.e., a distance between eachisland, of the first metal layer 4 is preferably equal to or larger thanthe mean diameter of the islands. The thickness of the first metal layer4 is preferably in a range from 5 to 100 Å and more preferably in arange from 10 to 50 Å. With this thickness, it is possible to form thefirst metal layer 4 with a fine pitch islands each having a microscopicdiameter, by using the cluster formation in sputtering. On the otherhand, the second metal layer 6 is formed to at least completely coverthe first metal layer 4. The thickness of the second metal layer 6 ispreferably 2000 to 5000 Å, more preferably in a range from 2500 to 3000Å.

The first metal layer 4 made of a silicon alloy is preferably formed bysputtering using a target made of a silicon alloy having an intendedcomposition. Accordingly, the density of the silicon substrate 2 can bemaintained substantially constantly even in the vicinity of theinterface with the metal electrode 8. For example, in a case where asilicon alloy having the composition TiSi₂ is formed as the first metallayer 4, it is possible that Ti is formed in island shape and thenannealing is performed to obtain TiSi₂. However, the silicon containedin the silicon substrate 2 is consumed to form a silicon alloy, so thatthe density of the silicon substrate 2 decreases in the vicinity of theinterface with the metal electrode 8. On the other hand, in a case wherethe first metal layer 4 is formed by sputtering using TiSi₂ as thetarget, the density of the silicon substrate 2 can be maintained also inthe vicinity of the interface with the metal electrode 8. Thus, it ispreferable if the density of the silicon substrate 2 is substantiallyconstant in the vicinity of the interface with the metal electrode 8,the ohmic contactivity and adhesion between the silicon substrate 2 andthe metal electrode 8 can be more improved.

(Method of Manufacturing Nitride Semiconductor Device 1)

Next, a method of manufacturing the nitride semiconductor device 1 willbe described with reference to FIGS. 5A to 5H. For illustrative purpose,the patterns of the p-electrode 12 and the n-electrode 16 are simplifiedin FIGS. 5A to 5H.

As shown in FIG. 5A, the nitride semiconductor layer 10 is formed on thesurface of the growth substrate 20 made of sapphire and the like. Thenitride semiconductor layer 10 is formed by stacking a plurality ofn-type and p-type nitride semiconductor layers to realize an appropriatedevice structure. In view of crystallinity, it is preferable to grow thep-type nitride semiconductor layer after growing the n-type nitridesemiconductor layer. For example, in the case of a nitride semiconductorlight emitting device, the device structure includes a buffer layer, ahigh-temperature growth layer, an n-cladding layer, an active layer, ap-type cladding layer, and a p-type contact layer and the like.

Next, as shown in FIG. 5B, the p-electrode 12 is formed to ohmicallyconnect to the p-side surface of the nitride semiconductor layer 10. Forexample, Rh and the like can be used for the p-electrode 12. Then, theinsulating protective film 14 such as SiO₂ is formed around thep-electrode 12, and the semiconductor side of the adhesion layer 15 suchas Ti, the semiconductor side of the barrier layer such as Pt 13, andthe first conductive bonding layer 17 a are sequentially formed from thenitride semiconductor layer 10 side to cover the entire surface of thedevice. The first conductive bonding layer 17 a contains a relativelylow melting point metal such as Sn, In as its main component. Inaddition, it is preferable to dispose an Au layer (not shown) at theinterface of exposed surface of the first conductive bonding layer 17 aand/or at the interface between the first conductive bonding layer 17 aand the semiconductor side of the barrier layer 13. The Au layerprotects the first conductive bonding layer 17 a and further improvesthe bonding.

Next, as shown in FIG. 5C, the p-type (or n-type) silicon substrate 2 isprepared, and thereon, the first metal layer 4 made of a silicon alloysuch as TiSi₂ is formed in a shape of discrete islands. The first metallayer 4 is preferably formed by sputtering using an appropriate siliconalloy target. That is, the sputtering is stopped before a uniform filmis obtained so that the first metal layer 4 can be formed in a shape ofdiscrete islands.

Next, as shown in FIG. 5D, a second metal layer 6 made of Pt and thelike is formed to cover the first metal layer 4 formed in a shape ofdiscrete islands. The second metal layer 6 can be formed by using aconventional method such as sputtering and vacuum deposition. Thus, ametal electrode 8 is formed on the silicon substrate 2.

Next, as shown in FIG. 5E, on the silicon substrate 2, a substrate sideof adhesion layer 9, a substrate side of barrier layer 11, and a secondconductive bonding layer 17 b are further formed, then the siliconsubstrate 2 is turned upside down. The second conductive bonding layer17 b is made of Pd, Au and the like and is capable of forming aneutectic crystal with the first conductive bonding layer 17 a. Thesecond conductive bonding layer 17 b contains a metal having a highermelting point than that of the first conductive bonding layer 17 a as amain component. The silicon substrate 2 is stacked on the nitridesemiconductor layer 10 so that the first conductive bonding layer 17 aand the conductive bonding layer 17 b are placed facing each other,then, heat pressure welding is carried out. The substrate side of theadhesion layer 9 and the substrate side of the barrier layer 11 are notnecessarily needed or can be omitted. In addition, the materials for thefirst conductive bonding layer 17 a and the second conductive bondinglayer 17 b are preferably selected from a combination of materials whichhave good wettability to each other and interdiffuse with each other.For example, combinations such as Au and Sn, Sn and Pd, Ag and Sn havegood wettability to each other and interdiffuse with each other.Therefore, for example, it is possible to form the first conductivebonding layer 17 a with Au—Sn alloy and the second conductive bondinglayer 17 b with Au or Pd.

As shown in FIG. 5F, when heat-pressed, the first conductive bondinglayer 17 a and the second conductive bonding layer 17 b interdiffuseinto each other to form a conductive bonding layer 17, so that thesilicon substrate 2 and the nitride semiconductor layer 10 are bonded.

Then, as shown in FIG. 5G, the device is turned upside down with thesilicon substrate 2 is on the downside, and is placed into a polishingmachine and wrapping of the growth substrate 20 is carried out. Withthis operation, the buffer layer and the underlayer in the nitridesemiconductor layer 10 can also be removed.

Next, as shown in FIG. 5H, after polishing the n-side surface of thenitride semiconductor layer 10, the n-electrode 16 of such asTi/Al/Ni/Au is formed thereon and the surface is covered with the secondinsulating film 18 except the pad portion. Next, on the back surface 24of the silicon substrate 2, the first metal layer 4′ made of TiSi₂ andthe like is disposed in a shape of discrete islands, and the secondmetal layer 6′ made of Pt and the like is formed to cover the firstmetal layer 4′. Thus, the metal electrode 8′ is formed. At this time, ifa high temperature greater than the melting point of the conductivebonding layer 17 is applied to the nitride semiconductor device, thesilicon substrate 2 may detach from the nitride semiconductor layer 10.If the second metal layer 6 is made of Pt and the like, for example, themetal electrode 8′ of the present embodiment is capable of forming ohmiccontact without performing a high temperature annealing. Therefore, themetal electrode 8′ with good ohmic contact property can be formedwithout causing difficulties such as detachment of the silicon substrate2.

In addition, the back surface 24 of the silicon substrate 2 may bepolished before forming the first metal layer 4′ and the second metallayer 6′. This enables to form a thin silicon substrate 2, so that athin nitride semiconductor device can be obtained. Particularly, it ispreferable when the thickness of the silicon substrate 2 is in a rangefrom 50 μm to 250 μm, a thin nitride semiconductor device can beobtained while securing a necessary thickness so as to serve as asubstrate for the nitride semiconductor device 2.

In order to reduce the thickness of the silicon substrate 2, it ispreferable to polish the back surface 24 of the silicon substrate 2after bonding the nitride semiconductor layer 10 to the siliconsubstrate 2. Polishing of the silicon substrate 2 before bonding thenitride semiconductor layer 10 thereto is undesirable. Because a thinsilicon substrate 2 has a low mechanical strength and polishing of thesilicon substrate 2 before bonding the nitride semiconductor layer 10may damage the silicon substrate 2. Further, if the thickness of thesilicon substrate is reduced before bonding with the nitridesemiconductor layer, the silicon substrate tends to be damaged due todifference in thermal expansion coefficient between the siliconsubstrate and the nitride semiconductor layer. Particularly, when thethickness of the silicon substrate 2 is reduced to 250 μm or less,possibility of damage caused by the above is increased. Therefore,polishing the back surface 24 of the silicon substrate 2 after bondingthe silicon substrate 2 with the nitride semiconductor layer 10 enablesto obtain a thin silicon substrate 2 without damaging the siliconsubstrate 2.

In addition, as shown in the flow-chart of FIG. 6, when polishing theback surface 24 of the silicon substrate 2, the metal electrode 8′ ofthe back surface 24 is needed to be disposed after the polishing processS20. That is, the process of forming the metal electrode 8 (the processS30 of forming the first metal layer 4′ and the process of forming thesecond metal layer 6′) is needed to be performed after the process ofbonding the silicon substrate 2 and the nitride semiconductor layer 10.The metal electrode 8′ of the present embodiment is capable of formingohmic contact without performing a high temperature annealing.Therefore, even if the metal electrode 8′ is disposed after bonding thesilicon substrate 2 and the nitride semiconductor layer 10, the metalelectrode 8′ having good ohmic contact property can be formed withoutcausing detachment and the like between the silicon substrate 2 and thenitride semiconductor layer 10.

Then, the light emitting device is divided into chips by dicing. Thus,obtained are the nitride semiconductor devices each having the structurein which the nitride semiconductor layer 10 is formed on the siliconsubstrate 2 and the n-electrode 16 and the p-electrode 12 are formed soas to place the semiconductor layer 10 in between. At this time, a forcesuch as stress and shock that may cause detachment is applied to themetal substrates 8, 8′ of the silicon substrate 2. However, the metalelectrodes 8, 8′ of the present embodiment have excellent adhesion, sothat detachment from the silicon substrate 2 can be preventedefficiently.

Next, structures and manufacturing process of the major components of anitride semiconductor device of the present embodiment will be describedin further detail.

(Silicon Substrate 2)

The silicon substrate 2 bonding to the nitride semiconductor layer 10may be of either n-type or p-type. The conductivity of the siliconsubstrate 2 is preferably in a range from 0.002 to 3 Ωcm. Theconductivity (and the dopant concentration) of the silicon substrate 2in this range facilitates forming ohmic contact with the metalelectrodes 8, 8′ and a nitride semiconductor device with a low Vf can beobtained. A p-type silicon substrate can be made p-type by, for example,B-doping, so that inexpensive CZ method can be employed. The resistivityof the p-type silicon substrate can also be controlled easier than inthe n-type silicon substrate.

(Substrate-Side Adhesion Layer 9, Semiconductor-Side Adhesion Layer 15)

Adhesion layers such as the substrate-side adhesion layer 9 and thesemiconductor-side adhesion layer 15 are to secure a high adhesion withthe underlayer and preferably contain at least one metal of Ti, Ni, W,and Mo. The thickness of the adhesion layer is preferably in a rangefrom 50 to 500 nm.

(Substrate-Side Barrier Layer 11, Semiconductor-Side Barrier Layer 13)

A barrier layer such as the substrate-side barrier layer 11 and thesemiconductor-side barrier layer 13 is to prevent diffusion of metalfrom the conductive bonding layer 17, and preferably containing a metalsuch as a platinum-group metal (Ru, Rh, Pd, Os, Ir, Pt), Ti, Ni, and W.Among platinum-group metals, Pt, Pd, and Rh are preferable, and Pt or Pdis particularly preferable. Pd is most preferable. In the barrier layer,diffusion of metal may be prevented by using a material which is eitherintermixable or not intermixable with the conductive bonding layer 17.An example of the former is a platinum-group metal, Ti, and Ni, and anexample of the latter is W. The thickness of the barrier layer ispreferably in a range from 100 to 1000 nm. The adhesion layer and/or thebarrier layer can be omitted according to the structure of the nitridesemiconductor device.

(First Conductive Bonding Layer 17 a)

The first conductive bonding layer 17 a contains a metal having arelatively low melting point as its main component. The metal preferablyhas a melting point of 300° C. and below and is capable of forming aeutectic crystal with the second conductive bonding layer 17 b whichwill hereinafter be described. For example, Sn or In is preferable andSn is particularly preferable. The thickness of the first conductivebonding layer 17 a is preferably in a range from 1000 to 3000 nm.

(Au Layer (Not Shown))

In addition, in order to prevent oxidation of the first conductivebonding layer 17 a, it is preferable to form a metal layer havingmelting point being higher than that of the first conductive bondinglayer 17 a and lower than that of the second conductive bonding layer 17b, for example, an Au layer, on the first conductive bonding layer 17 a.The Au layer is preferably disposed also between the semiconductor-sidebarrier layer 13 and the first conductive bonding layer 17 a, to furtherprevent diffusion of the first conductive bonding layer 17 a into thesemiconductor-side adhesion layer and the like.

(Second Conductive Bonding Layer 17 b)

The second conductive bonding layer 17 b contains a metal having arelatively high melting point as its main component. As the secondconductive bonding layer 17 b, a metal which is capable of forming aeutectic crystal with the first conductive bonding layer 17 a and havinga higher melting point than that of the first conductive bonding layeris selected. For example, the second conductive bonding layer 17 b ispreferably made of Pd or Au, and particularly, Pd is more preferable.The thickness of the second conductive bonding layer 17 b is preferably500 nm or below, more preferably 350 nm or below.

The present embodiment illustrates an example in which the firstconductive bonding layer 17 a is formed at the nitride semiconductorlayer 10 side and the second conductive bonding layer 17 b is formed atthe silicon substrate 2 side, but a structure having a reverseconstruction may be employed. The above described is also applicable insitu in that case, if the nitride semiconductor layer and the siliconsubstrate are considered interchanged each other.

(Nitride Semiconductor Layer 10)

The nitride semiconductor layer 10 is represented by general formulaAl_(a)In_(b)Ga_(1-a-b)N (0≦a≦1, 0≦b≦1, a+b≦1) and may have variousstructures according to the type of the device. For example, in the caseof the nitride semiconductor device, it is preferable to at leastinclude one or more p-type nitride semiconductor layers, an active layerhaving a quantum well structure at least including a well layer ofAl_(a)In_(b)Ga_(1-a-b)N (0≦a≦1, 0≦b≦1, a+b≦1) and a barrier layer ofAl_(c)In_(d)Ga_(1-c-d)N (0≦c≦1, 0≦d≦1, c+d≦1), and one or more an n-typenitride semiconductor layers.

(p-Electrode 12)

The p-electrode 12 preferably form ohmic contact with the p-type nitridesemiconductor layer and have a high reflectivity. A metal material whichcontain at least a metal selected from the group consisting of Ag, Rh,Ni, Au, Pd, Ir, Ti, Pt, W, and Al is preferably used for the p-electrode12. One of Rh, Ag, nickel-gold, Ni/Au/RhO and Rh/Ir is preferable, andRh is more preferable. The p-electrode 12 is formed on the p-typenitride semiconductor layer which has a higher resistivity than that ofthe n-type nitride semiconductor layer so that the p-electrode 12 ispreferably disposed over substantially entire surface of the p-side ofthe nitride semiconductor layer. In addition, the thickness of thep-electrode 12 is preferably in a range from 0.05 to 0.5 μm.

An opening is defined in the p-electrode 12 which is in contact with thenitride semiconductor 10 and the insulating protective film 14 isdisposed in the opening. A single-film or multi-film of SiO₂, Al₂O₃,ZrO₂, TiO₂ and the like can be used for the material of the insulatingprotective film 14. Use of the insulating protective film 14 enables toprevent occurrence of a short circuit and the like, so that processyield and reliability can be improved. The insulating protective film 14may form a two-layer structure with a reflective film (not shown). Forexample, when a reflective film (not shown) made of Al, Ag, or Rh, orthe like is formed with a thickness in a range from 500 Å to 2000 Å, ata side of the insulating protective film 14 which is not in contact withthe nitride semiconductor 10, light traveling in a lateral direction canbe extracted efficiently. The reflective film may be provided on eitherthe silicon substrate 2 side or the nitride semiconductor 10 side.

(n-Electrode 16)

A multi-layer electrode of Ti/Al/Ni/Au or W/Al/W/Pt/Au may be used asthe n-electrode 16 formed on the n-side of the nitride semiconductorlayer 10. The thickness of the n-electrode 16 is preferably in a rangefrom 0.1 to 1.5 μm. In addition, a second insulating protective film 18such as SiO₂, Al₂O₃, ZrO₂, or TiO₂ is preferably formed to cover theexposed surface except for the n-electrode.

In addition, as shown in FIGS. 1A and 1B, the n-electrode 16 has a padportion 16 a at each diagonal corner on a diagonal line across the chipportion and expands in a net shape between the pad portions 16 a. Whenthe n-electrode 16 is formed in a shape such as a net shape or a gridshape on the substantially entire surface of the light emitting region,the electric current can be supplied uniformly to the nitridesemiconductor layer 10. The pad portions 16 a may be formed not only attwo corners on the diagonal line but also at all of the four corners.The p-electrode 12 and the n-electrode 16 are preferably arrangedalternately at the opposite sides with respect to the nitridesemiconductor layer 10 in cross-sectional view. With this arrangement,emitted light can be extracted effectively without being interrupted bythe n-electrode 16.

(Growth Substrate 20)

Examples of the growth substrate for growing the nitride semiconductorlayer 10 include sapphire having any of C-plane, R-plane, and A-plane asa main surface, spinel (an insulating substrate of such as MgAl₂O₄),SiC, Si and an oxide substrate having lattice conformity with that ofthe nitride semiconductor. Sapphire and spinel are preferable.

(Removal Process of Growth Substrate 20)

Polishing, etching, electromagnetic irradiation, or a combined methodthereof can be used to remove the growth substrate 20 after bonding thesilicon substrate 2. When electromagnetic irradiation is used, a laser,for example, is used as an electromagnetic radiation source. Afterbonding the silicon substrate 2, laser is irradiated onto the entiresurface of the growth substrate 20 except the portion where theunderlayer is formed, then, the underlayer is decomposed. Thus, thegrowth substrate and the underlayer can be removed. The work can besimplified and process yield can be improved by using electromagneticirradiation, compared with a method by polishing. Further, after thegrowth substrate and the underlayer are removed, CMP is performed on theexposed surface of the nitride semiconductor layer to expose a desiredlayer. With this, a damaged layer can be removed and thickness andsurface roughness of the nitride semiconductor layer can be adjusted.

After the growth substrate 20 is removed, projected and recessed part(dimples) may be formed by RIE on the exposed surface of the nitridesemiconductor layer 10 to improve light extraction efficiency. Theprojected and recessed part (dimples) forming portion is situated at thelight extracting side of the nitride semiconductor. The angle of lightcan be changed by the projected and recessed face. Therefore, it becomespossible to extract light that underwent total reflection and wasconfined within the device. That is, emitted light is diffuselyreflected by the projected and recessed part, so that light that hadundergone total reflection can be directed upwardly to be extractedoutside the device. With the projected and recessed part, an improvementin the output power which is 1.5 times or more higher than that of adevice without having the projected and recessed part is expected. Theplanar shape of the projected and recessed part is preferably circularor polygonal such as hexagonal and triangular. In addition, the planarconfiguration of the projected and recessed part may be a stripe, agrid, rectangles, or the like.

(Fluorescent Material)

When the nitride semiconductor device is a light emitting device, lightof various wavelengths can be emitted by forming a coating layer or asealing member which contains a fluorescent material capable ofabsorbing a part or the whole of light emitted from the active layer andemitting light having a different wavelength than that of absorbedlight. The fluorescent material preferably absorbs a part of emissionfrom the active layer and converts it into light having a longerwavelength so that together with the light from the active layer, awhite light can be emitted.

In the present embodiment, a case in which a stacked layer of a nitridesemiconductor is bonded on a silicon substrate is illustrated, but thepresent invention is not limited thereto. An example may be such that astacked layer of a nitride semiconductor is grown directly on a siliconsubstrate and the electrode of the substrate side described above isformed on the back surface of the silicon substrate.

EXAMPLE 1

In the present example, the present invention is applied to a lightemitting diode having an emission wavelength of 375 nm, and according tothe method shown in FIGS. 5A to 5F, a nitride semiconductor devicehaving a structure shown in FIGS. 1A and 1B is fabricated.

(Growth Substrate)

A substrate made of sapphire (C-plane) was used as a growth substrate 20and surface cleaning was carrier out at 1050° C. in hydrogen atmospherein a MOOCVD reaction vessel.

(Underlayer)

Buffer layer: Successively, a buffer layer of GaN of about 200 Å inthickness was grown on the substrate at 510° C. in hydrogen atmosphereby using ammonia and TMG (trimethylgallium).

High temperature grown layer: After the buffer layer was grown, only TMGwas stopped and the temperature was raised to 1050° C., then a hightemperature grown nitride semiconductor 4 of undoped GaN was grown to athickness of 5 μm by using TMG and ammonia as source gases.

(n-Type Cladding/Contact Layer)

Next, an n-type cladding layer of n-type Al_(0.18)Ga_(0.82)N doped withSi in a concentration of 5×10¹⁷/cm³ was grown to a thickness of 400 Å byusing TMG, TMA, ammonia, and silane at 1050° C.

(Active Layer 6)

Next, at the temperature of 800° C. and using TMI (trimethylindium),TMG, and TMA as source gases, the barrier layers of Si-dopedAl_(0.1)Ga_(0.9)N and well layers of undopedIn_(0.03)Al_(0.02)Ga_(0.95)N were stacked in the order of barrier layer(1)/well layer (1)/barrier layer (2)/well layer (2)/barrier layer (3).In this case, the thickness was set to be 200 Å for the barrier layer(1), 40 Å for the barrier layers (2) and (3), and 70 Å for the welllayers (1) and (2). The active layer having a multiquantum well (MQW)structure with the total thickness of about 420 Å was grown.

(p-Type Cladding Layer)

Next, a p-side cladding layer 7 of Al_(0.2)Ga_(0.8)N doped with1×10²⁰/cm³ of Mg was grown to a thickness of 600 Å in a hydrogenatmosphere at 1050° C. by using TMG, TMA, ammonia, and Cp₂Mg(cyclopentadienyl magnesium).

(p-Type Contact Layer)

Then, a first p-type contact layer of Al_(0.04)Ga_(0.96)N doped with1×10¹⁹/cm³ of Mg was grown to a thickness of 0.1 μm on the p-typecladding layer by using TMG, TMA, ammonia, Cp₂Mg. Then, the gas flowrates were adjusted and a second p-type contact layer ofAl_(0.01)Ga_(0.99)N doped with 2×10²¹/cm³ of Mg was grown to a thicknessof 0.02 μm.

On completion of the growth, the wafer was annealed in a nitrogenatmosphere at 700° C. in the reaction vessel to further lower theresistance of the p-type cladding layer.

(p-Electrode 12)

After annealing, the wafer was taken out of the reaction vessel and a Rhfilm of 2000 Å in thickness was grown on the p-type contact layer toform a p-electrode 12. Then, ohmic annealing was carried out at 600° C.and then an insulating protective film 14 of SiO₂ was formed to athickness of 0.3 μm on the exposed surface except for the p-electrode12.

(Semiconductor-side Adhesion Layer 15, Semiconductor-side Barrier Layer13, First Conductive Bonding Layer 17 a)

Next, a multilayer film of Ti/Pt/Au/Sn/Au was formed on the p-electrodeto a thickness of 100 nm/500 nm/300 nm/3000 nm/100 nm. In this case, Tiwas the semiconductor-side adhesion layer 15, Pt was thesemiconductor-side barrier layer 13, Sn was a first conductive bondinglayer 17 a, and Au layer between Pt and Sn was a layer for preventingdiffusion of Sn to the barrier layer 44, and the outermost Au layer wasa layer for improving adhesion strength to a second conductive bondinglayer.

(Silicon Substrate 2)

On the other hand, the p-type silicon substrate of 400 μm in thickness,doped with boron, with resistance of 3 Ωcm or less was used as theconductive silicon substrate 2. The silicon substrate 2 was immersed inan organic solution such as acetone to remove an organic material fromits surface. Then, the silicon substrate 2 was further immersed in a HFaqueous solution to remove a native oxide film formed on the surface.Thus, the front surface 22 of the silicon substrate 2 was cleaned. Then,sputtering was carried out on the front surface 22 of the siliconsubstrate 2 using TiSi₂ as a target until the film thickness reached 30Å in growth rate equivalent. Thus, the fourth metal layer 4 made ofTiSi₂ in a shape of islands was formed. Then, sputtering was carried outusing Pt as a target to form a second metal layer 6 made of Pt with athickness of 250 Å. Thus, a metal electrode 8 was formed on the siliconsubstrate 2. Then, a second conductive bonding layer 17 b made of Pd wasformed thereon with a thickness of 350 nm.

Next, the first conductive bonding layer 17 a and the second conductivebonding layer 17 b are placed facing each other and heat pressurewelding was carried out at the heater temperature of 250° C. Thus, thefirst conductive bonding layer 17 a and the second conductive bondinglayer 17 b were diffused to each other and the conductive bonding layer17 was formed.

(Removal of Growth Substrate 20)

Next, after removing the sapphire substrate 1 by grinding, the exposedbuffer layer and the high temperature grown layer were polished untilthe n-type cladding layer was exposed, so as to substantially eliminatethe surface roughness.

(n-Electrode 16)

Next, a multilayer electrode of Ti/Pt/Au was disposed with a thicknessof 5 nm/100 nm/1800 nm on the n-type cladding layer functioning also asthe n-type contact layer to form the n-electrode 16. Then, the siliconsubstrate 2 is polished to a thickness of 100 μm, and a first metallayer 4′ of TiSi₂ having a shape of islands and a second metal layer 6′of Pt were formed on the back surface 24 of the silicon substrate 2 wereformed to a thickness of 2500 Å in the similar manner as in the frontsurface side 22 of the silicon substrate 2 to form a metal electrode 8′,then, an AU layer was further formed with a thickness of 5000 Å. Next,devices were separated by dicing.

The nitride semiconductor devices thus obtained exhibited excellentadhesion between the silicon substrate 2 and the metal electrodes 8, 8′.

The adhesion between the silicon substrate and the metal electrodes 8,8′ can be evaluated, for example, by modified Edge Liftoff Test (mELTmethod) proposed by KOBELCO Research Institute, Inc. In the modifiedEdge Liftoff Test, an epoxy resin is applied to a measurement sample.The sample is heated and diced into 10 mm square pieces and cooled withliquid nitrogen. The adhesion is determined by the temperature at whichdetachment of the film is observed. That is, the fracture toughnessK_(app) [MPa·m^(1/2)] is calculated based on the temperature T at whichdetachment occurs, the residual stress a of the epoxy resin, and thethickness h of the epoxy resin.

K_(aap)=σ·(h/2)^(1/2)

wherein, the residual stress a of the epoxy resin at temperature T canbe calculated by the formula as follows based on Cf, m, and c of theepoxy resin.

σ=C _(f) T ² −mT−c

TiSi₂, Pt layer of 1200 Å, Au layer of 1200 Å, and Ti layer of 100 Å (toenhance adhesion with an epoxy resin) are formed on the siliconsubstrate in the similar manner as in Example 1. The fracture toughnessK_(app) was measured with varying the thickness of TiSi₂ in a rangebetween 0 Å and 100 Å. The fracture toughness of the sample in whichTiSi₂ was not formed (thickness of 0 Å) was about 0.15 [MPa·m^(1/2)]. Onthe other hand, the fracture toughness K_(app) of each of the samples inwhich TiSi₂ was formed was approximately 0.3 [MPa·m^(1/2)], which isabout twice the improvement in the fracture toughness.

EXAMPLE 2

A nitride semiconductor device is formed in a similar manner as inExample 1, except that the first metal layer is NiSi₂. The nitridesemiconductor device having significantly excellent adhesion between thesilicon substrate and the metal electrodes 8,8′ is obtained similarly asin Example 1.

EXAMPLE 3

A nitride semiconductor device is formed in a similar manner as inExample 1, except that the p-type silicon substrate is used in place ofthe n-type silicon substrate and the second metal layer 6 is Ti. Thenitride semiconductor device having significantly excellent adhesionbetween the silicon substrate and the metal electrodes 8,8′ is obtainedsimilarly as in Example 1.

EXAMPLE 4

In Example 4, a device is fabricated using a process shown in FIG. 6.Similarly as in Example 1, the back surface 24 of the silicon substrate2 is polished to reduce the thickness of the silicon substrate 2 from400 μm to 100 μm. In this example, subsequently, a process for polishingthe back surface 24 is performed. Then, after the polishing process, themetal electrode 8′ is formed. As compared with the case of Example 1,the nitride semiconductor device having further improved adhesionbetween the back surface 25 of the silicon substrate 2 and the metalelectrode 8′ is obtained.

Comparative Example 1

In a case where the TiSi₂ layer is not formed as islands but formed as acontinuous layer at a thickness of 100 Å, and the Pt film is formed at athickness of 1200 Å, ohmic connection between the silicon substrate andthe TiSi₂ layer is not obtained. Accordingly, such a device as will bedescribed below is fabricated. A nitride semiconductor device forcomparison was formed in the same manner as in Example 1 except that themetal electrodes 8, 8′ are changed. In this comparative example, acontinuous TiSi₂ layers are formed at a thickness of 100 Å on the frontsurface 22 and the back surface 24 of the silicon substrate 2 in placeof island-shaped first metal layers 4, 4′. Then, Pt films are formed ata thickness of 1200 A in place of the second metal layers 6, 6′. The Ptfilm is formed on the TiSi₂ layer, so that it is not in direct contactwith the silicon substrate 2.

Ohmic contact between the silicon substrate 2 and TiSi₂ layer cannot beformed in the device for comparison thus obtained, so that its drivingpower is substantially greater than that in the device according toExample 1. This device for comparison has a high driving power, evenwhen one of the metal electrode 8 at the front surface side 22 or themetal electrode 8′ at the back surface side 24 of the silicon substrate2 is changed to an electrode including a TiSi₂ layer and a Pt film.

Comparative Example 2

A nitride semiconductor device for comparison was formed in the samemanner as in Example 1 except that the metal electrode 8′ was changed.In this comparative example, a continuous TiSi₂ layer was formed with100 Å thickness on the back surface 24 of the silicon substrate 2 inplace of island-shaped first metal layers 4′. Subsequently, the siliconsubstrate 2 is annealed at 600° C. and TiSi₂ layer of 200 Å thicknesswas formed from the Ti layer. Then, a Pt film was formed with athickness of 1200 Å in place of the second metal layer 6′. The Pt filmwas formed on the TiSi₂ layer and was not in direct contact with thesilicon substrate 2.

In this method, detachment may occur in the vicinity of the firstconductive bonding layer 17 a and the second conductive bonding layer 17b disposed between the silicon substrate 2 and the nitride semiconductorlayer 10, which then cause detachment of the silicon substrate from thenitride semiconductor layer, so that a device cannot be obtained.

It is to be understood that although the present invention has beendescribed with regard to preferred embodiments thereof, various otherembodiments and variants may occur to those skilled in the art, whichare within the scope and spirit of the invention, and such otherembodiments and variants are intended to be covered by the followingclaims.

This application is based on applications No. 2007-143208 filed in Japanon May 30, 2007, and No. 2008-138074 filed in Japan on May 27, 2008, thecontents of which are incorporated hereinto by reference.

1. A nitride semiconductor device comprising: a silicon substrate; anitride semiconductor layer formed on the silicon substrate; and a metalelectrode formed in contact with the silicon substrate; wherein themetal electrode has a first metal layer which is in contact with thesilicon substrate and is formed in a shape of discrete islands, and asecond metal layer which is formed in contact with the silicon substrateexposed among the islands of the first metal layer and which covers thefirst metal layer, and wherein the second metal layer is made of a metalcapable of establishing ohmic contact with silicon and the first metallayer is made of an alloy containing a metal and silicon, in which themetal is different than that in the second metal layer.
 2. The nitridesemiconductor device according to claim 1, wherein the first metal layeris made of an alloy containing a transition metal and silicon.
 3. Thenitride semiconductor device according to claim 1, wherein the firstmetal layer is made of an alloy containing at least an element selectedfrom Ti, W, Co, and Ni, and silicon.
 4. The nitride semiconductor deviceaccording to claim 1, wherein the mean diameter of the islands of thefirst metal layer is 100 Å or less.
 5. The nitride semiconductor deviceaccording to claim 1, wherein thickness of the first metal layer is in arange from 5 to 100 Å.
 6. The nitride semiconductor device according toclaim 1, wherein the silicon substrate is of a p-type and the secondmetal layer contains at least one element of the platinum group as amajor component.
 7. The nitride semiconductor device according to claim6, wherein the second metal layer contains Pt as a major component. 8.The nitride semiconductor device according to claim 1, wherein thesilicon substrate is of an n-type and the second metal layer contains Tior Al as a major component.
 9. The nitride semiconductor deviceaccording to claim 1, wherein the silicon substrate and the nitridesemiconductor layer are bonded by a conductive bonding layer.
 10. Thenitride semiconductor device according to claim 1, wherein the metalelectrode is formed on a first main surface of the silicon substrate andthe nitride semiconductor layer is formed on the metal electrode. 11.The nitride semiconductor device according to claim 1, wherein the metalelectrode is formed on a second main surface of the silicon substrateand the nitride semiconductor layer is formed on the first main surfacefacing the second main surface.
 12. The nitride semiconductor deviceaccording to claim 10, wherein the metal electrode is also formed on thesecond main surface of the silicon substrate.
 13. A method ofmanufacturing a nitride semiconductor device having a silicon substrate,a nitride semiconductor layer formed on the silicon substrate, and ametal electrode formed on the silicon substrate comprising: a process offorming a metal electrode, the process including; a step of forming afirst metal layer on the silicon substrate in a shape of discreteislands, and a step of forming a second metal layer so as to be incontact with the silicon substrate exposed among the first metal layerhaving discrete island-shape and to cover the first metal layer, thesecond metal layer being made of a metal capable of establishing ohmiccontact with silicon, and the first metal layer being made of an alloyincluding a metal which is different than the second metal layer andsilicon.
 14. The method of manufacturing the nitride semiconductordevice according to claim 13, wherein in the process of forming a metalelectrode, the metal electrode is formed on a second main surface facingthe first main surface.
 15. The method of manufacturing the nitridesemiconductor device according to claim 14 further including a processof bonding the silicon substrate and the nitride semiconductor layerbefore the process of forming a metal electrode.
 16. The method ofmanufacturing the nitride semiconductor device according to claim 15,further including a process of polishing the second main surface of thesilicon substrate between the process of bonding and the process offorming a metal electrode.
 17. The method of manufacturing the nitridesemiconductor device according to claim 16, wherein in the process ofpolishing, the silicon substrate is polished to 50 μm to 250 μm inthickness.
 18. The method of manufacturing the nitride semiconductordevice according to claim 13, wherein in the step of forming a firstmetal layer, the first metal layer is formed with an alloy containing atransition metal and silicon.
 19. The method of manufacturing thenitride semiconductor device according to claim 13, wherein in the stepof forming a first metal layer, the islands of the first metal layer areformed with the mean diameter of 100 Å or less.
 20. The method ofmanufacturing the nitride semiconductor device according to claim 13,wherein the silicon substrate is of p-type, and in the step of forming asecond metal layer, the second metal layer is formed with a materialcontaining a main component of at least one platinum group metal.